In the manufacturing of integrated circuits, grinding is a commonly used technology. In a grinding process, a grinding wheel is placed over a wafer. The grinding wheel and the wafer both rotate, so that the thickness of the wafer is reduced due to the removal of the surface layers by the grinding wheel.
In the manufacturing of device wafers, grinding may be used in the backside thinning of silicon substrates, for example, in the formation of through-silicon vias (TSVs). There are other processes in which the grinding technology may be used. In the formation of fan-out chip scale packages, device wafers may be sawed, and the known-good-dies are selected and attached onto a carrier, with the known-good-dies spaced apart from each other. The known-good-dies include copper posts for the formation of fan-out connections. A molding compound is then filled into the space between and over the known-good-dies to form a fan-out wafer. After the curing of the molding compound, a grinding process may be performed to remove the portions of the molding compound and other dielectric materials over the copper posts. After the copper posts are exposed, electrical connections may be made to connect to the copper posts, so that the connections to the fan-out wafer are extended into an area larger than the area of the known-good-dies.
Since the layers that are subject to the grinding are often thin layers, accurately stopping the grinding process on the right time is vital to the yield of the integrated manufacturing process. For example, in the manufacturing of the fan-out wafer, the grinding needs to be stopped when the copper posts in substantially all known-good-dies throughout the fan-out wafer are fully exposed, and substantially no over-grinding occur. In the existing grinding technology, a gauge is used to detect the total thickness of the fan-out wafer during the grinding process. When the total thickness is reduced to a pre-determined value, it is assumed that the copper posts are fully exposed. This detection method, however, is inaccurate, and may result in yield loss.